Modulation is the fundamental process in any communication system. It is a process to impress a message (voice, image, data, etc.) on to a carrier wave for transmission. A band-limited range of frequencies that comprise the message (baseband) is translated to a higher range of frequencies. The band-limited message is preserved, i.e., every frequency in that message is scaled by a constant value. The three key parameters of a carrier wave are its amplitude, its phase and its frequency, all of which can be modified in accordance with an information signal to obtain the modulated signal.
There are various shapes and forms of modulators. For example conventional Amplitude Modulation uses a number of different techniques for modulating the amplitude of the carrier in accordance with the information signal. These techniques have been described in detail in “Modern Analog and Digital Communication Systems” by B. P. Lathi. Similarly conventional Frequency/Phase Modulation uses a number of different methods described in a number of textbooks. In all these techniques, carrier (which is a high frequency sinusoidal signal) characteristics (either amplitude, frequency, phase, or combination of these) are changed in accordance with the data (or information signal). Thus there have been two major components of a modulator. One is the information-carrying signal and the other is the high frequency carrier. An unconventional modulator is described in this document that does not use a carrier for modulation. Modulation is accomplished by exploiting the impulse response of Band Pass Filters.
In a communication system, band pass filters are used to band limit the bandwidth of the signal. For example, they are used in transmitters to allow necessary signal to pass to the next stage and in receivers they are used to block any unwanted signal. They are integral part of any communication system and have numerous advantages. Band Pass filters come in many shapes and forms. Most of the communication systems these days use SAW (Surface Acoustic Wave) filters. Saw filters are band pass filters. They use a piezoelectric crystal substrate with deposited gold electrodes. SAW filters are capable of replacing discrete LC band pass filters in certain wideband applications between 20 MHz and 1 GHz. Their filter skirts, or shape factor are the sharpest of all the filter structures. Since they are etched on a printed circuit board, they save a lot of circuit board real estate and are thus easier to implement. The primary use of SAW filters (as the name implies) is to filter unnecessary signals such as band limiting a transmitter output. A technique for using SAW filters as modulators has been described by the inventor in another patent titled “Carrier-less Modulator using SAW filters”, U.S. Application No. 60/781,718, however, the current modulation technique is significantly different than the one described in the earlier patent application. A series of narrow pulses (representing one modulation event) is applied to the input of the SAW filter. The SAW filter, being a resonant device, rings at the fundamental frequency thus modulating the incoming pulse train without using a carrier for modulation.
Communication systems that have emerged in recent years include monopulse and Ultra-Wide Band communication systems. The problem with these systems is that all monopulse or Ultra-Wide Band communications systems form Power Spectrum Densities that tend to span very wide swaths of the radio spectrum. For instance the FCC has conditionally allowed limited power use of UWB from 3.2 GHz to 10 GHz. These systems must make use of very wide sections of radio spectrum because the transmit power in any narrow section of the spectrum is very low. Generally any 4 KHz section of the affected spectrum will contain no more than −42 dbm of UWB spectral power. Correlating receivers are used to “gather” such very wide spectral power and concentrate it into detectable pulses. Interfering signals are problematic. Since the communication system is receiving energy over a very wide spectrum, any interfering signal in that spectrum must be tolerated and mitigated within the receiver. Many schemes exist to mitigate the interference. Some of these include selective blocking of certain sections of spectrum so as not to hear the interferer, OFDM schemes that send redundant copies of the information in the hope that at least one copy will get through interference, and other more exotic schemes that require sophisticated DSP algorithms to perform advanced filtering. In addition, UWB systems have somewhat of a “bad reputation” because they at least have the potential to cause interference. A heated discourse has gone on for years over the potential that UWB systems can cause interference to legacy spectrum users.
Tri-State Integer Cycle Modulation (TICM) and other Integer Cycle Modulation techniques were designed by Joe Bobier and the inventor of this application to help alleviate this massive and growing problem which has now become known by its commercial designation, xG Flash Signaling. Its signal characteristics are such that absolute minimal sideband energy is generated during modulation but that power spectrum density is quite wide relative to the information rate applied. Also, a narrower section of the power spectrum output can be used to represent the same information. The technique of modulation disclosed herein is primarily applicable to these types of single cycle systems.
Like any other band pass filters, SAW filters also have an impulse response. The impulse response depends on the bandwidth of the filter. A technique is described in this disclosure that uses the impulse response of the filter to modulate the incoming data signal without using a carrier for modulation by exciting the filter with a narrow pulse train producing an impulse radio system with limited bandwidth, low average power, but high peak power.